The present invention relates to an electrolyte for use in electrolyte circulation-type zinc-bromine storage batteries and more particularly to the composition of a negative electrolyte capable of inhibiting the formation of zinc dendrites.
Zinc-bromine storage batteries have been studied and developed from old times for their use in practical applications in view of their high energy densities and in recent years their development for practical applications has been spotlighted again from the standpoint of effective energy utilization. For instance, in FIG. 1 showing a basic construction of an electrolyte circulation-type zinc-bromine storage battery, numeral 1 designates a unit cell, 2 a positive electrode chamber, 3 a negative electrode chamber, 4 a separator (e.g., an ion exchange membrane or fine porous membrane separator), 5 a positive electrode, 6 a negative electrode, 7 a positive electrolyte, 8 a negative electrolyte, 9 a positive electrolyte storage tank, 10 a negative electrolyte storage tank, and 11 and 12 pumps.
With this type of electrolyte circulation-type zinc-bromine storage battery, as shown in FIG. 2, during the charge the zinc deposited on the surface of the negative electrode 6 designated at .crclbar. in the Figure frequently tends to become uneven and cause the formation of tree-like crystals or so-called dendrites 13 due to nonuniformity of the electric field and disturbance in the flow of the negative electrolyte in the vicinity of the negative electrode surface, and in particular the following problems will be caused as the number of charge and discharge cycles is increased. The deposited dendritic zinc is very fragile so that it tends to fall off the electrode and deteriorate the energy efficiency of the battery. Even if the dendrites do not fall off the electrode, the dendrite 13 continues to grow in a branching form so that it penetrates and breaks the separator 4 and a short-circuit is formed between it and the positive electrode 5 thus eventually causing damage to the battery.
While various kinds of inhibitors, such as, nonionic surface active agents, zinc electrochemical plating brighteners, etc., have been used for the purpose of preventing the above-mentioned dendrite formation, due to the fact that these agents have relatively small effects when used singly and that an inexpensive fine porous membrane is usually used for the separator 4, during the charge the bromine produced at the positive electrode penetrates through the fine-porous membrane separator and diffuses within the negative electrode chamber. As a result, from the standpoint of resistance to the bromine there are problems that the dendrite inhibitor is gradually decomposed due to the strong oxidation of the bromine making it impossible to maintain the stable performance for the cyclic use over a long period of time and so on. In addition, as compared with the lead storage battery, an electrical resistance of the electrolyte is large in the zinc-bromine storage battery so that while many different kinds of electrolyte additives have been conceived for improving the voltage efficiency, their use in practical applications has required that the proper concentration of each of such additives be grasped in consideration of the interaction with the dendrite inhibitor used.
The inventors, etc., have investigated into the causes for the formation of dendrites in the electrolyte circulation-type zinc-bromine storage battery and have discovered and invented a negative electrolyte composition capable of suppressing the formation of dendrites during the charge of the zinc-bromine storage battery as the result of repeated tests performed by adding to the negative electrolyte mixtures of the negative electrolyte active material, i.e., zinc bromide, specific amounts of various bromine complexing agents and suitable amounts of various dendrite inhibitors and electric conductivity improvers in accordance with the results of the investigations.